Starch constitutes an important source of macro-molecules since it is renewable by means of the photosynthesis cycle, and it is available in large amounts, is biodegradable and is inexpensive.
Owing to its chemical and physical structure, starch is not intrinsically a thermoplastic and it is mainly used in water-based preparations as a binder, thickener or foodstuff.
Attempts have therefore been made to modify starch by plastifying it and/or by subjecting it to an appropriate heat treatment so as to render it workable at high temperature.
The use of a starch which is structurally modified during a preliminary processing such as an extrusion and, optionally, the addition of water, which renders the final product mouldable at high temperature, has been proposed in European Patent Application EP-A-0 304 401 (Warner Lambert Co.).
Additional components such as plasticisers comprising polyalkylene glycol or glycerol acetate can also be incorporated into structurally modified starch.
The use of another structurally modified starch, whose viscosity has been reduced by means of a depolymerisation catalyst, is proposed in European Patent Application EP-A-0 282 451 (Warner Lambert Co.).
Additional components such as plasticisers, diluents or fillers can also be incorporated into starch.
The extrusion of thermoplastic starch in the form of a mixture of a starch, which is at least partially crystalline, with water and/or other additives such as glycerine (5 to 35% by weight relative to the total weight of the mixture), which reduce its melting point (see page 27, lines 1 to 7 and claim 13), is known from the document WO-90 10 019 (Tomka).
A technique for heating and mixing starch with other additives which permit melting of the starch at a temperature lower than its decomposition temperature, is known from patent application WO-90 05 161 (Tomka).
However, starch which is treated by these processes remains a hydrophilic material which absorbs water, which plasticises it and causes it to loose all mechanical properties. This constitutes a major obstacle to a mass use of this product, although the hydrophilic character also constitutes a property which is essential for good biodegradability.
Research studies have therefore also been carried out in order to reduce the sensitivity of starch to water, especially by coating the starch with a hydrophobic compound.
European Patent Application EP-0 378 646-A (Tomka) (equivalent to WO-90 00 1043) describes the coating of a hydrophilic polymer such as starch or gelatine with a film-forming hydrophobic compound consisting of polyhydroxycarboxylic acid, optionally supplemented with proteins, natural lipids, wax and/or paraffin.
The document "Starch-based film for Degradable Agricultural Mulch" (Ind. Eng. Chem. Prod. Res. Develop. vol. 13 (1), 1974, p. 90) describes a coating with polyvinyl chloride (PVC) or with a vinylidene chloride and acrylonitrile copolymer, of a film of starch supplemented with polyvinyl alcohol (PVA) and cross-linked with formaldehyde.
Mixtures of starch with copolymers have also been described in the following documents:
U.S. Pat. No. 4,133,784 (U.S. Secretary of Agriculture) and the document "Composites of starch and polyethylene coacrylic acid, complexing between polymer components", (Journal of Applied Polymeric Science, vol. 40, pp. 811 to 821, 1990) mention a composite material based on starch and an ethylene and acrylic acid (EAA) copolymer which is described as being stable under atmospheric conditions only for limited periods of time.
Patent EP-0 408 503-A (Warner Lambert Co.) describes a composite material consisting of structurally modified starch and ethylene-vinyl alcohol type (EVOH) copolymers and optionally a thermoplastic polymer which is insoluble in water.
However, the products obtained are found to be fragile and tear easily directly after their processing, and during storage they soften with the absorption of water and the plasticisation which it produces.